1. Field of the Invention
The present invention relates to electrographic development apparatus and, more particularly, to development apparatus for presenting an electrically conducting developing material to a surface bearing a latent electrostatic image.
2. Description of the Prior Art
A number of techniques are known in the art for developing latent electrostatic charge patterns carried on an image-bearing support surface. In one widely used technique, a developer mixture is formed by mixing finely divided, pigmented, insulating toner particles with relatively larger carrier particles. The surface characteristics of these materials are selected to be different so that surface electrical charges of mutually opposite polarity are produced on the toner particles and carrier particles creating a physically attractive force between them. To produce a visible image, the developer mixture is presented to a latent electrostatic charge pattern on an image-bearing support surface. In the charged areas, the toner particles are separated from the triboelectrically attracted carrier particles by the stronger forces associated with the electrostatic image. However, in the uncharged areas, no toner particles are deposited because the toner particles remain physically attracted to the carrier particles. Developing systems using two component developer mixtures have a number of detractive aspects including a high cost of manufacture, the need to maintain the ratio of the developer mixture within narrow limits, a tendency for the mixture to agglomerate and a limitation as to the form of image produced, i.e., negative or positive.
Another known developing technique contacts the latent electrostatic charge pattern with single component, electrically conductive developer particles. When a charged area is so contacted, an opposite polarity charge is induced to the developer particles, in an amount which is directly proportional to field strength, creating an electrically attractive force sufficient to hold one or more particles to the charge-bearing surface. In the uncharged areas no charges are induced and therefore no toner particles are attracted. However, due to Van der Waal's and other forces, a large number of toner particles contacting the background areas of the image remain adhering creating an undesirable background haze in the copies produced. When the electrostatic charge pattern is created by uniformly charging a photoconductor in darkness and then exposing the photoconductor to an imagewise light pattern, the attractive forces exerted by the residual voltages present in the light exposed, background areas increase the density of toner background deposition. Another disadvantage of conductive developing systems is their tendency to create an airborne toner cloud which contaminates the machine components thereby necessitating frequent cleaning. The latter problem is known in the art as "toner dusting."
Background toner depositation can be controlled by the use of a free-flowing, magnetically responsive, electrically conductive developer powder in combination with a magnetic applicator. By properly adjusting the magnetic field strength, the magnetic force of attraction between the developer particles and the applicator is greater than the forces of attraction which may exist between the developer particles and the background areas of the image pattern bearing surface. As a result, the developer particles remain adhered to the surface of the applicator when the applicator is removed from the background areas. In the charged image areas, however, the magnetic force of attraction between the developer particles and the applicator is less than the electrical force of attraction between the particles and the charge-bearing surface. The particles are, therefore, separated from the magnetic applicator and transferred to the charge-bearing surface. While effective in reducing toner background deposits, a magnetic development system has the obvious disadvantage of increased complexity and cost.